1. Field of the Invention
The present invention relates to a semiconductor device, a method of producing the semiconductor device, a liquid crystal display device, and a method of producing the liquid crystal display device, and more specifically, to a semiconductor device that achieves high reliability, a method of producing the semiconductor device, a liquid crystal display device, and a method of producing the liquid crystal display device.
2. Description of the Background Art
Conventionally, a liquid crystal display device utilizing a thin film field-effect transistor is known to be one type of a liquid crystal display device. A glass substrate on which the thin film field-effect transistor is formed in such a liquid crystal display device is shown in FIG. 38. FIG. 38 is a schematic cross sectional view showing a conventional liquid crystal display device. The liquid crystal display device will be described with reference to FIG. 38.
As shown in FIG. 38, an n-type thin film field-effect transistor 119 and a p-type thin film field-effect transistor 120 are formed in a drive circuit region on a glass substrate 101 of the liquid crystal display device. In addition, a capacitance 121 and a thin film field-effect transistor for a pixel 122 are formed in a display pixel region.
An underlying film 102 is formed on glass substrate 101 in the drive circuit region. A silicon oxide film is used as the underlying film. On underlying film 102, n+ type impurity regions 103a, 103b, nxe2x88x92 type impurity regions 104a, 104b, and a channel region 106a are formed using the same semiconductor film. An insulating film 107 serving as a gate insulating film is formed on channel region 106a. A gate electrode 108a is formed on gate insulating film 107. N+ type impurity regions 103a, 103b and nxe2x88x92 type impurity regions 104a, 104b form source/drain regions. N+ type impurity regions 103a, 103b, nxe2x88x92 type impurity regions 104a, 104b, channel region 106a, insulating film 107 located on channel region 106a, and gate electrode 108a formed an n-type thin film field-effect transistor 119.
In addition, on underlying film 102, p-type impurity regions 105a, 105b and a channel region 106b are formed using the same semiconductor film. Insulating film 107 serving as a gate insulating film is formed on channel region 106b. A gate electrode 108b is formed on insulating film 107 in the region located above channel region 106b. P-type impurity regions 105a, 105b, channel region 106b, insulating film 107 serving as the gate insulating film, and gate electrode 108b form a p-type thin film field-effect transistor 120. An interlayer insulating film 110 is formed on n-type thin film field-effect transistor 119 and p-type thin film field-effect transistor 120. Contact holes 111a to 111d are formed in regions located above n+ type impurity regions 103a, 103b and p-type impurity regions 105a, 105b by removing parts of interlayer insulating film 110 and insulating film 107. Metal interconnections 112a to 112d are formed such that they extend from inside contact holes 111a to 111d onto an upper surface of interlayer insulating film 110. A passivation film (not shown) is formed on metal interconnections 112a to 112d. A planarized film 113 is formed on the passivation film.
In the display pixel region, a capacitance electrode 109 is formed on underlying film 102. Another capacitance electrode 108e is formed on capacitance electrode 109 with insulating film 107 that serves as a dielectric film existing therebetween. Capacitance electrodes 109, 108e and insulating film 107 form capacitance 121. An n+ type impurity region 103c serving as a conductive region is formed on underlying film 102 adjacent to capacitance electrode 109.
Moreover, on underlying film 102, n+ type impurity regions 103d to 103f, nxe2x88x92 type impurity regions 104d to 104g, and channel regions 106c, 106d are formed using the same semiconductor film. Gate electrodes 108c, 108d are formed on channel regions 106c, 106d with insulating film 107 serving as a gate insulating film existing therebetween. Thus, n+ type impurity regions 103d, 103e, nxe2x88x92 type impurity regions 104d, 104e, channel region 106c, insulating film 107 serving as the gate insulating film, and gate electrode 108c form one thin film field-effect transistor. In addition, n+ type impurity regions 103e, 103f, nxe2x88x92 type impurity regions 104f, 104g, channel region 106d, insulating film 107 serving as the gate insulating film, and gate electrode 108d form another thin film field-effect transistor. Thin film field-effect transistor for a pixel 122 includes these two thin film field-effect transistors.
Interlayer insulating film 110 is formed on capacitance 121 and thin film field-effect transistor for a pixel 122. Contact holes 111e to 111g are formed in regions located above n+ type impurity regions 103c, 103d, 103f by removing parts of interlayer insulating film 110 and insulating film 107. Metal interconnections 112e, 112f are formed such that they extend from inside contact holes 111e to 111g onto an upper surface of interlayer insulating film 110. A passivation film (not shown) is formed on metal interconnections 112e, 112f. Planarized film 113 is formed on the passivation film. A contact hole 114 is formed in planarized film 113 and the passivation film in the region located above metal interconnection 112e. A pixel electrode 115 using ITO (Indium Tin Oxide) and the like is formed such that it extends from inside contact hole 114 onto an upper surface of planarized film 113.
FIGS. 39 to 42 are schematic cross sectional views related to the description of a method of producing the liquid crystal display device shown in FIG. 38. The method of producing the liquid crystal display device will be described with reference to FIGS. 39 to 42.
First, underlying film 102 such as a silicon oxide film is formed on glass substrate 101. An amorphous silicon film 126 is formed on underlying film 102. Thus, the structure as the one shown in FIG. 39 can be obtained.
Then, amorphous silicon film 126 is annealed using a laser or the like to produce a polysilicon film 128. As a result, a structure such as the one shown in FIG. 40 is obtained.
Then, a resist film (not shown) is formed on polysilicon film 128. The resist film is subjected to exposure and development processes so as to form a channel pattern. Then, polysilicon film 128 is partially removed by etching using as a mask the resist film in which the channel pattern is formed so as to form polysilicon film 129a to 129d (see FIG. 41). Thereafter, the resist film is removed. Thus, the structure as the one shown in FIG. 41 is obtained.
Then, as shown in FIG. 42, a resist film 130 is formed in a region other than the region in which polysilicon film 129c is formed. Conductive impurities are implanted into polysilicon film 129c that is to become a capacitance electrode using resist film 130 as a mask to form capacitance electrode 109. Phosphorus ions 131 are used as the conductive impurities to be implanted at this time. Thereafter, resist film 130 is removed.
Then, insulating film 107 (see FIG. 38) is formed on polysilicon film 129a, 129b, 129d and capacitance electrode 109. A conductor film is formed on insulating film 107. A resist film is formed on this conductor film. The resist film is subjected to exposure and development processes so as to form a gate pattern. The conductive film is partially removed by etching using as a mask the resist film in which this gate pattern is formed so as to form gate electrodes 108a to 108d (see FIG. 38) and a capacitance electrode 108e (see FIG. 38). Thereafter, the resist film is removed. A resist film is formed so as to cover the region in which a p-type thin film field-effect transistor 120 (see FIG. 38) is to be formed, and, at the same time, a resist film that is to serve as a mask for forming n+ type impurity regions 103a to 103f (see FIG. 38) is formed so as to cover gate electrodes 108a, 108c, and 108d. Then, phosphorus ions as dopant impurities are implanted into prescribed regions of polysilicon film 129a, 129d and capacitance electrode 109. Thus, n+ type impurity regions 103a to 103f are formed. Thereafter, the resist film is removed.
Then, phosphorus ions are implanted into prescribed regions of polysilicon film 129a, 129d using gate electrodes 108a, 108c, 108d as a mask to form n-type impurity regions 104a, 104b, 104d to 104g (see FIG. 38). Then, a resist film is formed in a region other than the region in which p-type thin film field-effect transistor 120 is to be formed. Thereafter, boron ions, which are p-type conductivity dopant impurities are implanted into a prescribed region of polysilicon film 129b using gate electrode 108b as a mask so as to form p-type impurity regions 105a, 105b and a channel region 106b (see FIG. 38). Thereafter, the resist film is removed.
Then, interlayer insulating film 110 (see FIG. 38) is formed on gate electrodes 108a to 108d and capacitance electrode 108e. A resist pattern is formed on interlayer insulating film 110. Contact holes 111a to 111g (see FIG. 38) are formed by removing parts of interlayer insulating film 110 and insulating film 107 using this resist pattern as a mask. Thereafter, the resist pattern is removed. Then, after a washing step is performed, a metal layer that is to become metal interconnections 112a to 112f (see FIG. 38) is formed so as to extend from inside contact holes 111a to 111g onto an upper surface of interlayer insulating film 110. A resist pattern is formed on the metal layer. The metal layer is partially removed by wet etching using the resist pattern as a mask. Thus, metal interconnections 112a to 112f are formed. Thereafter, the resist pattern is removed.
A passivation film is formed on metal interconnections 112a to 112f. A planarized film 113 (see FIG. 38) is formed on the passivation film. After an upper surface of planarized film 113 is planarized, a contact hole 114 (see FIG. 38) is formed in planarized film 113 and the passivation film. A transparent conductor film such as an ITO film is formed to extend from inside contact hole 114 onto the upper surface of planarized film 113. A resist film in which a pixel pattern is formed is formed on the transparent conductor film. The transparent conductor film is partially removed by wet etching using the resist film as a mask to form a pixel electrode 115 (see FIG. 38). Thereafter, the resist film is removed. Thus, the structure such as the one shown in FIG. 38 is obtained.
In the step shown in FIG. 40, amorphous silicon film 126 (see FIG. 39) is annealed by irradiation with light from laser 127 and becomes a polysilicon film 128. This laser annealing step involves the following problems.
FIG. 43 is a schematic diagram related to the description of the conventional problems. As shown in FIG. 43, in some cases, contaminated regions 160a to 160d may exist in which impurities (contaminants) such as boron and arsenic or sodium and sulfur exist in such locations as on a surface of amorphous silicon film 126, in the boundary region between amorphous silicon film 126 and underlying film 102, in the boundary region between underlying film 102 and glass substrate 101, or inside glass substrate 101 and so on. When the laser annealing step as shown in FIG. 40 is performed with such contaminated regions 160a to 160d present, impurities from these contaminated regions 160a to 160d invade polysilicon film 128 during the laser annealing step, thereby creating the problem of increased impurity concentration m polysilicon film 128.
Conventionally, in order to solve such problems, it is suggested to employ a technique of forming a barrier layer made of a silicon nitride film between polysilicon film 128 and glass substrate 101 so as to prevent the introduction of impurities such as alkali ions from glass substrate 101. This barrier layer, however, has no effect on the impurities in the boundary portion between the barrier layer and amorphous silicon film 126 or on the impurities existing on the upper surface of amorphous silicon film 126. In other words, the above-described technique does not provide a solution to the above-described problems on a fundamental level.
Moreover, when polysilicon film 128 is contaminated by impurities after the laser annealing step, the presence of the impurities results in holes or electrons in a concentration greater than the set value being supplied in the channel region. As a result, threshold voltage fluctuation of a thin film field-effect transistor occurs, for instance, causing the electrical characteristics of the thin film field-effect transistor to change.
FIGS. 44 and 45 are schematic diagrams related to the description of the conventional problems. When a region 161 of an upper surface of polysilicon film 128 after the laser anneal step in FIG. 44 is partly enlarged, it can be seen that, in some cases, a silicon protrusion 137 is formed on a surface of polysilicon film 128 during the laser anneal step. The height of protrusion 137 from the substantially flat upper surface of polysilicon film 128 at times could become higher than the thickness of polysilicon film 128. With such protrusion 137 formed on the surface of polysilicon film 128, when insulating film 107 serving as a gate insulating film is formed on polysilicon film 128, the thickness of insulating film 107 becomes locally thin in the region located above protrusion 137. When the thickness of insulating film 107 serving as the gate insulating film becomes thin locally in this manner, dielectric breakdown is likely to occur in the portion having a small film thickness. As a result, the reliability of the thin film field-effect transistor is degraded. Moreover, such degradation of the reliability of the thin film field-effect transistor has caused a reduction in the yield of the liquid crystal display devices utilizing the thin film field-effect transistors as well as a variation in display characteristics in the display pixel region.
One object of the present invention is to provide a highly reliable semiconductor device and a method of producing such a semiconductor device.
Another object of the present invention is to provide a liquid crystal display device that achieves a high yield and that has good display characteristics and a method of producing such a liquid crystal display device.
According to one aspect of the present invention, a semiconductor device having a thin film field-effect transistor including a channel region includes a substrate and a semiconductor film. The semiconductor film is formed on the substrate, and includes the channel region of the thin film field-effect transistor. The semiconductor film has an upper surface that is planarized by the removal of a surface layer of the semiconductor film.
In this manner, the upper surface of the semiconductor film is planarized. Thus, the problem of the thickness of an insulating film serving as a gate insulating film formed on the channel region becoming locally thin due to the presence of a protrusion, as well as, the concentration of the electric field can be prevented. Consequently, the reduction in the dielectric strength of the gate insulating film can be prevented. As a result, a highly reliable semiconductor device can be obtained.
In addition, in the step of forming the semiconductor film, the impurities in the semiconductor film can be removed with certainty by concentrating or segregating the impurities within the semiconductor film into the surface layer and by removing this surface layer. In this manner, the threshold voltage fluctuation of a thin film field-effect transistor due to the impurities within the semiconductor film can be prevented. As a result, a semiconductor device having a thin film field-effect transistor of higher reliability can be obtained.
In the semiconductor device according to the above-described one aspect, an upper surface of the semiconductor film may include a protruded portion, and a height of the protruded portion from the substantially flat upper surface of the semiconductor film is preferably lower than a thickness of the semiconductor film.
If the semiconductor film is left as it is after the laser anneal processing, a protrusion having a height greater than the thickness of the semiconductor film might be formed on the upper surface of the semiconductor film. By removing the surface layer of the semiconductor film and planarizing the semiconductor film, however, the height of the protrusion (protruded portion) can be made lower than the thickness of the semiconductor film. Thus, a proportion of a decrease in the thickness of the gate insulating film formed on the semiconductor film can be reduced, and a concentration of the electric field can be prevented. As a result, the probability of dielectric breakdown in the gate insulating film can be reduced.
According to another aspect of the present invention, a semiconductor device having a thin film field-effect transistor including a channel region includes a substrate and a semiconductor film. The semiconductor film is formed on the substrate, includes a channel region of the thin film field-effect transistor, and is formed into a polycrystal by laser annealing. An upper surface of the semiconductor film includes a protruded portion. The height of the protruded portion from a substantially flat upper surface of the semiconductor film is lower than the thickness of the semiconductor film.
In this manner, a protruded portion having a relatively low height is formed on the upper surface of the semiconductor film so that, when a gate insulating film is formed on the semiconductor film, the proportion of the decrease in the thickness of the gate insulating film due to the presence of the protruded portion can be reduced, while the concentration of the electric field can be prevented. Consequently, the probability of dielectric breakdown caused by such reduction in the thickness of the gate insulating film can be decreased. As a result, a highly reliable semiconductor device can be obtained.
The liquid crystal display device according to a further aspect of the present invention includes a semiconductor device according to the above-described one aspect or the above-described another aspect.
Thus, high reliability of the liquid crystal display device can be achieved by using a thin film field-effect transistor of high reliability according to the present invention in a drive circuit region or a display pixel region of the liquid crystal display device, and at the same time, the probability of the defective products being produced can be reduced so that the yield can be improved. In addition, application of such a highly reliable semiconductor device as a pixel in the display pixel region improves the uniformity of the liquid crystal display screen.
According to a still further aspect of the present invention, a method of producing a semiconductor device having a thin film field-effect transistor including a channel region involves forming an amorphous semiconductor film on a substrate. The amorphous semiconductor film is subjected to a heat treatment to form a crystalline semiconductor film including a region that is to become the channel region. A surface layer of the crystalline semiconductor film is removed.
In this manner, even when a defect of form such as a protrusion is produced on a surface of the semiconductor film due to a heat treatment, such a defect of form portion can be removed by the removal of the surface layer of the crystalline semiconductor film. Thus, when a gate insulating film of a thin film field-effect transistor is formed on the semiconductor film, the problem of the thickness of the gate insulating film becoming locally thin due to the protrusion on the surface of the semiconductor film and the concentration of the electric field can be prevented. Consequently, a problem such as dielectric breakdown of the gate insulating film due to the thickness of the gate insulating film becoming locally thin can be prevented. As a result, a highly reliable semiconductor device can be obtained.
In the method of producing a semiconductor device according to the above-described still further aspect, a step of forming a crystalline semiconductor film preferably includes a high purity achieving step in which impurities within the semiconductor film are concentrated or segregated into a surface layer of the crystalline semiconductor film.
In this case, by performing a step of removing the surface layer, impurities within the semiconductor film concentrated or segregated into the surface layer can be removed from the semiconductor film at the same time. Thus, a semiconductor film having a low impurity concentration can be obtained. As a consequence, the problem of the threshold voltage fluctuation of a thin film field-effect transistor owing to impurity concentration in a channel region becoming unnecessarily high can be positively prevented. As a result, a highly reliable semiconductor device can be obtained.
In the method of producing a semiconductor device according to the above-described still further aspect, the high purity achieving step preferably includes lowering a temperature in a region adjacent to a substrate in an amorphous semiconductor film below a temperature of an upper surface layer in the amorphous semiconductor film during the heat treatment of the amorphous semiconductor film.
In this case, crystallization of the semiconductor film progresses from the region of the semiconductor film adjacent to the substrate during the heat treatment. An upper surface layer of the amorphous semiconductor film would be the last to undergo the crystallization in the semiconductor film. Moreover, impurities such as boron and arsenic concentrate or segregate in a grain boundary or the like of such region in which crystallization takes place last (surface layer of the crystalline semiconductor film). Thus, removal of the surface layer ensures removal of the impurities from the crystalline semiconductor film.
In the method of producing a semiconductor device according to the above-described still further aspect, the high purity achieving step preferably includes applying an electric field to the amorphous semiconductor film during heat treatment of the amorphous semiconductor film.
In this case, impurities can move within the semiconductor film relatively easily during the heat treatment of the amorphous semiconductor film. When impurities exist as ions having positive or negative electric charges, an electric field can be applied substantially in the vertical direction to an upper surface of the semiconductor film with the direction of the electric field controlled so as to move the impurity ions having charges of a desired sign in a direction of a surface layer of the semiconductor film. In other words, it becomes possible to accelerate the concentration or the segregation of the impurity ions into the surface layer of the semiconductor film. In addition, the impurity ions that are positive or negative can be selectively concentrated or segregated to the surface layer of the semiconductor film according to the sign of charge so that the impurity ions can be removed from the semiconductor film selectively according to the sign of electric charge.
In the method of producing a semiconductor device according to the above-described still further aspect, the crystalline semiconductor film is preferably subjected to a further heat treatment while applying an electric field of a direction that is opposite to the direction of the electric field in the above high purity achieving step to the crystalline semiconductor film whose surface layer is removed. A surface layer of the crystalline semiconductor film subjected to the further heat treatment is preferably further removed.
Let us consider the case in which the impurity ions having positive charges, for instance, are concentrated or segregated into the surface layer of the semiconductor film in the initial high purity achieving step and thereby removed. By performing the further heat treatment while applying the electric field of the opposite direction to that in the initial high purity achieving step, the impurity ions having negative charges can be concentrated or segregated into the surface layer of the semiconductor film. The further removal of the surface layer of the semiconductor film subjected to the further heat treatment ensures the removal of these impurity ions having negative charges from the semiconductor film. As a result, the removal of both the impurity ions having positive charges and the impurity ions having negative charges from the semiconductor film can be ensured. Thus, a decrease in concentration of impurities within the semiconductor film can be ensured so that such problems as a threshold voltage fluctuation of the thin film field-effect transistor due to these impurities can be prevented.
In the method of producing a semiconductor device according to the above-described still further aspect, the high purity achieving step preferably includes applying a magnetic field to the amorphous semiconductor film during the heat treatment of the amorphous semiconductor film.
In this case, when a substrate on which the amorphous semiconductor film is formed is subjected to the heat treatment, the substrate is moved in the horizontal direction, for instance, and a magnetic field which is parallel to the surface of the substrate and vertical to the moving direction of the substrate is applied to the amorphous semiconductor film. In this manner, the impurity ions would move according to the movement of the substrate so that the impurity ions would be subjected to a force (Lorentz""s force) in a direction substantially vertical to the surface of the semiconductor film or a thickness direction of the amorphous semiconductor film. As a result, as in the case in which an electric field is applied described above, it becomes possible to concentrate or segregate the impurity ions having charges of a desired sign into the surface layer of the semiconductor film.
In addition, although a device such as an electrode need to be arranged above the amorphous film when an electric field is to be applied, a device for applying a magnetic field can be arranged in a region other than above the substrate when the magnetic field is to be applied in the horizontal direction of the substrate as described above. Consequently, no extra device is arranged above the substrate so that a laser beam and the like can be irradiated onto the semiconductor film with ease. As a result, heat treatment such as laser annealing can be performed easily.
In the method of producing a semiconductor device according to the above-described still further aspect, the crystalline semiconductor film is preferably subjected to a further heat treatment while applying a magnetic field in a direction opposite to the direction of the magnetic field in the high purity achieving step to the crystalline semiconductor film whose surface layer is removed. A surface layer of the crystalline semiconductor film subjected to the further heat treatment is preferably further removed.
In this case, a magnetic field of the direction opposite to the magnetic field in the above-described high purity achieving step is applied during the further heat treatment of the semiconductor film so that the impurity ions having charges of the conductivity type reverse of the impurity ions concentrated or segregated into the surface layer of the semiconductor film in the high purity achieving step can be concentrated or segregated into a surface layer of the semiconductor film. Thus, the removal of both the impurity ions having positive charges and the impurity ions having negative charges within the semiconductor film from the semiconductor film can be ensured.
In the method of producing a semiconductor device according to the above-described still further aspect, the high purity achieving step preferably involves applying a centrifugal force to the amorphous semiconductor film during the heat treatment of the amorphous semiconductor film.
In this manner, by applying a centrifugal force in a direction substantially vertical to a surface of the semiconductor film, the impurities can be easily concentrated or segregated into the surface layer of the semiconductor film due to a difference in specific gravities of material forming the semiconductor film and the impurities. For instance, when the amorphous semiconductor film is subjected to the heat treatment and attains a molten state, the centrifugal force applied in a direction toward the substrate from the upper surface of the semiconductor film can cause the impurities having a smaller specific gravity than the material forming the semiconductor film to concentrate or segregate into an upper surface side (surface layer) of the semiconductor film within the semiconductor film in the molten state. If the surface layer of the semiconductor film is removed thereafter, the removal of an impurity element having a relatively small specific gravity from the semiconductor film can be ensured. Moreover, by reversing the direction in which the centrifugal force is applied, impurities having a greater specific gravity than the material forming the semiconductor film can be easily concentrated or segregated into the surface layer of the semiconductor film. In other words, according to the direction in which the centrifugal force is applied, either the impurities having a smaller specific gravity or the impurities having a greater specific gravity than the material forming the semiconductor film can be selectively removed.
In the method of producing a semiconductor device according to the above-described still further aspect, the crystalline semiconductor film is preferably subjected to a further heat treatment while applying a centrifugal force in a direction that is opposite to the direction of the centrifugal force in the above-described high purity achieving step to the crystalline semiconductor film whose surface layer is removed. A surface layer of the crystalline semiconductor film subjected to the further heat treatment is preferably further removed.
In this case, the removal of both the impurities having a relatively small specific gravity and the impurities having a relatively large specific gravity with respect to the material forming the semiconductor film from the semiconductor film can be ensured. As a result, the threshold voltage fluctuation of the thin film field-effect transistor due to the presence of the impurities can be positively prevented.
In the method of producing a semiconductor device according to the above-described still further aspect, in the high purity achieving step, at least two steps are preferably performed during heat treatment of the amorphous semiconductor film which are selected from the group consisting of lowering the temperature in the region adjacent to the substrate in the amorphous semiconductor film below the temperature of an upper surface layer in the amorphous semiconductor film, applying an electric field to the amorphous semiconductor film, applying a magnetic field to the amorphous semiconductor film, and applying a centrifugal force to the amorphous semiconductor film.
In this case, a plurality of measures are implemented in order to concentrate or segregate the impurities in one high purity achieving step so that the impurities can be concentrated or segregated into the surface layer of the semiconductor film with greater certainty. As a result, the reduction in the impurity concentration within the semiconductor film can be ensured.
In the method of producing a semiconductor device according to the above-described still further aspect, prior to the step of removing a surface layer of the crystalline semiconductor film, a semiconductor film portion including a region to be the channel region and including a surface layer of the semiconductor film is preferably formed by partially removing the crystalline semiconductor film by etching.
Let us consider the case in which a resist film to be used as an etching mask is formed in the step of forming the semiconductor film portion. After forming the semiconductor film portion, even if such resist film remains on the semiconductor film portion, the remaining portion of such resist film can be removed along with a surface layer of the semiconductor film since the step of removing the surface layer of the semiconductor film is performed after the step of forming the semiconductor film portion. Consequently, such resist film is prevented from remaining on the semiconductor film portion. As a result, a structural defect of a thin film field-effect transistor due to the remaining resist film as described above can be prevented.
In addition, when impurities or foreign particles exist in a region other than the region in which the semiconductor film portion is formed after the step of forming the semiconductor film portion, these foreign particles can also be removed during the step of removing the surface layer of the semiconductor film. As a result, the prevention of a structural defect of a semiconductor device due to the presence of such foreign particles can be ensured.
In the method of producing a semiconductor device according to the above-described still further aspect, the step of forming the semiconductor film portion preferably includes a step of forming another semiconductor film portion by partially removing the semiconductor film by etching, and conductive impurities are preferably implanted into the above another semiconductor film portion prior to the step of removing the surface layer of the semiconductor film.
At this time, let us consider the case in which a resist film is formed on the semiconductor film as a mask for a step of implanting the conductive impurities. In such a case, this resist film at times could remain on the semiconductor film even after the step of removing the resist film. According to the present invention, however, the surface layer of the semiconductor film is removed after the step of implanting the conductive impurities so that the remaining portion of the resist film can be removed at the same time as the surface layer of the semiconductor film. Consequently, such problem as the remaining resist film can be positively prevented. As a result, a structural defect of a semiconductor device due to the remaining resist film can be prevented.
Moreover, in some cases, a defect could be produced in the surface layer of the semiconductor film owing to such steps as implanting the conductive impurities and removing the resist film as described above. Since the surface layer of the semiconductor film is removed after the step of implanting the conductive impurities, however, such defect portion in the surface layer of the semiconductor film can be removed from the semiconductor film. As a result, the degradation in the electrical characteristics of a thin film field-effect transistor due to the presence of such defect portion in the semiconductor film can be prevented.
According to an even further aspect of the present invention, a method of producing a semiconductor device having a thin film field-effect transistor including a channel region involves forming an amorphous semiconductor film on a substrate. An impurity absorption film is formed on the amorphous semiconductor film. The amorphous semiconductor film with the impurity absorption film formed thereon is subjected to a heat treatment to form a crystalline semiconductor film including a region that is to become the channel region. The impurity absorption film is removed.
In this manner, the impurity absorption film is formed in advance so that a portion of the semiconductor film need not be removed after concentrating or segregating the impurities. In other words, the deposited film thickness of the amorphous semiconductor film can be made thinner than that in the method of producing a semiconductor device according to the above-described still further aspect. Moreover, when employing a laser for the heat treatment, an output (power) of the laser needs to be increased in proportion to the film thickness of the amorphous semiconductor film to be subjected to the heat treatment; however, the heat treatment using a laser having a relatively small output becomes possible when the thickness of the amorphous semiconductor film is reduced. As a result, a reduction in the manufacturing cost of the semiconductor device can be achieved.
In the method of producing a semiconductor device according to the above-described even further aspect, prior to the step of removing the impurity absorption film, a semiconductor film portion including a region that is to become the channel region is preferably formed by partially removing the crystalline semiconductor film and the impurity absorption film by etching.
When a resist film is used as a mask in the etching for forming the semiconductor film portion, in some cases, the resist film may remain on the semiconductor film upon the removal of the resist film after the etching. Since the impurity absorption film is removed after forming the semiconductor film portion, however, the resist film remaining on the impurity absorption film can be removed at the same time. As a result, the problem of a structural defect of a thin film field-effect transistor due to the remaining resist film can be prevented.
In addition, the impurity absorption film is removed after the step of forming the semiconductor film portion so that, even when foreign particles and the like exist in the region other than the region in which the semiconductor film portion remains, these foreign particles can be removed at the same time in the step of removing the impurity absorption film. As a result, the defect of a semiconductor device due to the presence of such foreign particles can be prevented.
In the method of producing a semiconductor device according to the above-described even further aspect, the step of forming the semiconductor film portion preferably includes a step of forming another semiconductor film portion by partially removing the crystalline semiconductor film and the impurity absorption film by etching, and prior to the step of removing the impurity absorption film, a step of implanting conductive impurities into the above another semiconductor film portion is preferably provided.
Now, let us consider the case in which a resist film is formed as a mask to be used in the step of implanting the conductive impurities. In such a case, this resist film at times could remain on the semiconductor film and the like when the resist film is removed after the step of implanting the conductive impurities. The presence of such remaining resist film at times could cause a structural defect in the semiconductor device. By removing the impurity absorption film after the step of implanting the conductive impurities, however, the remaining portion of the resist film can be removed at the same time as the removal of the impurity absorption film. As a result, the prevention of a structural defect of the semiconductor device due to the remaining resist film can be ensured.
In addition, an ashing processing or the like normally performed in such step of removing the resist film can cause damage to the semiconductor film portion in some cases. The impurity absorption film, however, serves as the protection film to prevent the damage to the semiconductor film portion from such ashing process. Thus, the prevention of a defect in the semiconductor film portion caused by such ashing process can be ensured.
According to another still further aspect of the present invention, a method of producing a liquid crystal display device employs the method of producing a semiconductor device according to the above-described still further aspect or the method of producing a semiconductor device according to the above-described even further aspect.
In this manner, a liquid crystal display device having a semiconductor device such as a highly reliable thin film field-effect transistor and the like can be easily obtained.
Moreover, the application of such a highly reliable thin film field-effect transistor to a drive circuit or a display pixel of the liquid crystal display device improves the reliability of the liquid crystal display device and, at the same time, achieves uniformity of display characteristics of the liquid crystal display device. Furthermore, since highly reliable thin film field-effect transistors are used, the yield of the liquid crystal display devices can be improved.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.